For any kind of pollution, the prime polluter has incentive to spread factoids pointing to significant other polluters. The comment you respond to mentions fis

For any kind of pollution, the prime polluter has incentive to spread factoids pointing to significant other polluters. The comment you respond to mentions fishing nets to be the biggest pollutant by volume (which is quite logical if one thinks about it). Obviously a marine source has incentive to highlight landbased sources such as rivers. If one hears about the most polluting river one is easily confused into thinking rivers are the biggest polluter. Also note the questions surrounding metric of river pollution (i.e. per capita pollution of rivers and so on ...)